a transport layer approach for improving end-to-end performance

8/6/2019 A Transport Layer Approach for Improving End-To-End Performance

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A Transport Layer Approach

for Improving End-to-EndPerformance and RobustnessUsing Redundant Paths

Ming Zhang, Junwen Lai, Randy Wang, LarryPeterson

Princeton University

Arvind KrishnamurthyYale University

USENIX’04

Presented by Feng-Chiao Tseng



Outline

Introduction

mTCPTransport Layer Protocol

Path Selection

Sending PacketsShared Congestion Detection

Path Management

Path Failure Detection and Recovery

ImplementationEvaluation

Conclusion



Introduction(cont.)

ChallengesThe traditional congestion control mechanism inTCP needs to be modified

separate congestion control for each path

Paths may failpath failure detection and recovery

Paths may share congestionshared congestion detection and path suppression

There may exist many alternative pathspath selection



Transport Layer Protocol

Similar with TCPOrdered delivery

Congestion and flow control

One send/recv buffer, sequence number

Differenceseparate congestion control

Estimating outstanding packets for each pathModified fast retransmit

Stripe packets across multiple paths




Separate congestion controlEach subflow maintains a congestion window

(cwndi)

To minimize the negative influence of subflowson poor path




Estimating Outstanding packetsEach path maintain a pipei to represent thenumber of outstanding packets on pathi

Sender scoreboard records the outgoing pathsand status for each packet

Outstanding Packets on Path 1

1 2 3 4 5 Sequence Number

Outstanding Packets on Path 2



Path selection

Use Resilient Overlay Networks (RON) toobtain multiple paths

With n nodes, there are m=n-1 paths

RON uses the throughput score to representthe quality of each path

Select k disjoint paths from m paths

Robustness: less likely to fail simultaneouslyPerformance: less likely to share congestion



Path selection

Use traceroute to obtain router IPs along a pathand the latency of each physical link

Two paths X=(x0, x1, … xm) and Y=(y0, y1, …, yn)

L is the set of overlapping links of X and Y

X and Y are disjoint if Disjoint(X, Y) > β

∑∈

= Ll

latency(l)Y)Overlap(X,

) Latency(Y) y(X),min(LatencY)Overlap(X,1Y) , Disjoint(X −=



Path selection

Use greedy algorithmto select k disjointpaths from m paths

Start

Pick the highest score

Disjoint fromother selected?

Select it

Having k pathsor tried all path

End

N

N

Y

Y



Sending Packets

When to send a packet?There exists one path whose pipei<cwndi

Which packet to send?Normally sndnxt is the next packet to send

Retransmit any lost packets firstWhich path to use to send a packet?

Choose path with minimum scorei = pipei /cwndi

Original and retransmitted packets could be senton different pathsAdapt to dynamic variation in path characteristics



Sending Packets

All ACKs return over the same path

Simple, no states maintained by receiver

Avoid ACK reorderings, which increase burstiness of sender

Use the best path



Shared Congestion Detection

Aggressiveness Problem

mTCP subflows with shared congestion obtain unfairlarger bandwidth than one flow

Sol:suppress paths with shared congestion

Using fast retransmits to detect

two fast retransmit sequences S(s1, s2, …, sm) and T(t1,t2, …, tn)

Si matchs t j if |si – t j| < intervalMatch(S, T)

Share congestion if Match(S, T) / min(m, n) > δ(=0.5)



Shared Congestion Detection

How to choose interval?Too large

Too small

95% of congestion period < 220ms



Path Management

Path suppressionShare congestion

Throughput is too low

Path failure

MPd flowUse at least d paths

Larger d means better performance andreliability

Smaller d means less aggressive



Path Management

Path additionmTCP will periodically update the pathinformation by querying RON

Use a new path if it has higher score than apath being used



Path Failure Detection andRecovery

Failure RecoveryWhen primary path fails, all paths appear to fail

Send probe packet over all active pathsStop timers until a new primary path is selected

Receiver elects a new primary path

time

ti now

pathi pathpTimerp=Ip =4

62 4

now- Ip



Implementation

Implemented at the user-level

Composed of a user-level TCP/IP stack andan overlay router/forwarder modified fromRON

Add SACK options to user-level TCP

Communicate with RON through UDPsocket

Provide standard TCP socket interface toapplication



Evaluation

Run on PlanetLab and EmulabUtilizing Multiple Independent Paths



Evaluation



Evaluation

Shared Congestion DetectionInterval=200ms

δ=0.5

Run on a pair of paths for 60 seconds

Each experiment is repeated three times

Use the Princeton and Berkeley nodes as sourceand destination



Evaluation



Evaluation

Comparison with Single-path Flows

0

0.5

1

1.5

2

2.5

3

1 2 3 4 5 6

M B y t e s / s RON

INET

MP1



Conclusion

mTCP is a transport layer protocol forimproving end-to-end throughput androbustness

Aggregate available bandwidth on multiplepaths in parallel

Shared congestion detection and pathsuppression to alleviate aggressiveness

a transport layer approach for improving end-to-end performance

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